1. Field of the Invention
The present invention relates to piezoelectric electroacoustic transducers such as a piezoelectric receiver, a piezoelectric sounder, and a piezoelectric loudspeaker, and more particularly, the present invention relates to a surface-mountable electroacoustic transducer.
2. Description of the Related Art
Conventional electroacoustic transducers have been widely used in electronic apparatuses, household electrical appliances, portable phones, and so forth, to provide a piezoelectric sounder or a piezoelectric receiver generating an audible alarm or an operating sound.
The known electroacoustic transducer has a general structure in which a unimorph piezoelectric diaphragm is formed by affixing a piezoelectric plate onto one surface of a metal plate, the perimeter of the metal plate is fixed inside a casing by adhesion, and the opening of the casing is covered with a cover.
However, since such a diaphragm generates bending vibrations by restraining the piezoelectric plate generating square-type vibrations with the metal plate having an area that does not vary, the diaphragm has a low acoustic conversion efficiency and also has difficulties in having a compact structure and a sound characteristic having a low resonant frequency. In addition, the periphery of the diaphragm is restrained by the casing, causing a problem of a higher resonant frequency.
Japanese Unexamined Patent Application Publication No. 61-161100 has proposed a piezoelectric loudspeaker having a structure in which a round unimorph piezoelectric diaphragm is affixed onto the central portion of a round synthetic resin film. The film has a flat portion formed at the central portion thereof and has a circular projection formed around the flat portion by molding.
This proposed electroacoustic transducer has an advantage that a broader frequency characteristic than that of the above-described electroacoustic transducer formed by directly bonding the diaphragm to the casing is obtained due to the elasticity of the film and the projection.
However, because of a unimorph piezoelectric diaphragm, the diaphragm has difficulties in achieving high acoustic conversion efficiency and a compact structure. Also, since the diaphragm and the film are both round, their deformed volumes are small, thereby resulting in an unsatisfactory acoustic conversion efficiency.
Japanese Unexamined Patent Application Publication No. 2002-10393 has proposed a piezoelectric diaphragm having a high acoustic conversion efficiency. This piezoelectric diaphragm has a structure in which a laminate is formed by laminating two or three rectangular piezoelectric ceramic layers, having an internal electrode interposed between two of them, and has principal-surface electrodes formed on the front and rear principal surfaces thereof. The ceramic layers are polarized in the same thickness direction thereof, and, by applying an alternating signal between the principal-surface electrodes and the internal electrode, the laminate generates bending vibrations so as to generate a sound.
The piezoelectric diaphragm having the above-described structure is a ceramic laminate, and the two vibrating regions (ceramic layers) disposed one by one in the thickness direction vibrate in the opposite direction relative to each other, thereby achieving a greater deformation, that is, a higher sound pressure, than that achieved by a unimorph piezoelectric diaphragm in which a piezoelectric plate is affixed onto a metal plate. Also, this piezoelectric diaphragm is rectangular, thereby achieving a greater deformed volume and thus a higher sound pressure than those achieved by a round diaphragm.
Although the piezoelectric diaphragm has an excellent acoustic conversion efficiency as described above, this diaphragm has a problem of a high resonant frequency caused by its structure in which, when it is supported by a casing or the like, its surrounding area must be sealed by adhesion without leaving a space therein. For example, when two mutually opposed sides of the piezoelectric diaphragm having dimensions of 10 mm×10 mm are fixed onto to the casing by adhesion, and the other two sides are elastically sealed so as to be deformable, its resonant frequency lies at about 1200 Hz, thereby resulting in a significantly lowered sound pressure at about 300 Hz which is the lower limit of the human voice band.
A piezoelectric receiver requires an electroacoustic transducer that has an almost flat sound-pressure characteristic in a frequency band from 300 Hz to 3.4 kHz, which is equivalent to the human voice band, and that is capable of playing back a broadband voice. Unfortunately, the above-mentioned supporting structure does not permit the transducer to have an almost flat sound-pressure characteristic in a broad band. Although the larger casing and diaphragm lead to a lower resonant frequency, this results in a larger size of the electroacoustic transducer.
To solve the above-described problem, when the piezoelectric diaphragm generating surface bending-vibrations has a resin film that is larger than the piezoelectric diaphragm, affixed onto one surface thereof, and the outer periphery of the film is bonded to a support of a housing, the piezoelectric diaphragm can be supported without being strongly restrained. In this case, the piezoelectric diaphragm is more likely to vibrate than in the conventional case where two or four sides of the piezoelectric diaphragm are supported by the housing. As a result, even when the diaphragm has the same dimensions as those of the conventional one, its resonant frequency can be made lower, and also its deformation can be made greater because of a lowered support-constraining force exerted thereon, thereby achieving a high sound pressure. In addition, the obtained sound pressure does not drop in a frequency region from the fundamental resonant frequency to the secondary resonant frequency, thereby playing back of a broadband voice.
On the contrary, in the electroacoustic transducer having the above-mentioned resin film used therein, a stress exerted on the film varies in accordance with the bonding states between the film and the housing, thereby causing the diaphragm to have a shifted resonant frequency and accordingly a fluctuated frequency characteristic.
Although the electroacoustic transducer is also expected to be surface-mountable so as to be directly mounted on a circuit board, the film, the housing, an adhesive, and the like are deformed due to heat during reflow soldering, thereby causing a stress exerted on the piezoelectric diaphragm to vary and thus its frequency characteristic to vary before and after reflow soldering.